Add Msg.db_receiver_external field. Resolve #2193.

docs/source/Components/Msg.md

@@ -1,4 +1,4 @@
-# Msg 
+# Msg
 
 The [Msg](api:evennia.comms.models.Msg) object represents a database-saved
 piece of communication. Think of it as a discrete piece of email - it contains
@@ -29,10 +29,10 @@ good uses for `Msg` objects:
   Channels dropped Msg-support. Now only used in `page` command by default.
 ```
 
-## Msg in code 
+## Msg in code
 
 The Msg is intended to be used exclusively in code, to build other game systems. It is _not_
-a [Typeclassed](./Typeclasses) entity, which means it cannot (easily) be overridden. It 
+a [Typeclassed](./Typeclasses) entity, which means it cannot (easily) be overridden. It
 doesn't support Attributes (but it _does_ support [Tags](./Tags)). It tries to be lean
 and small since a new one is created for every message.
 
@@ -40,7 +40,7 @@ You create a new message with `evennia.create_message`:
 
 ```python
     from evennia import create_message
-    message = create_message(senders, message, receivers, 
+    message = create_message(senders, message, receivers,
                              locks=..., tags=..., header=...)
 ```
 
@@ -61,22 +61,26 @@ You can search for `Msg` objects in various ways:
 
 ### Properties on Msg
 
-- `senders` - there must always be at least one sender. This is one of [Account](./Accounts), [Object](./Objects), [Script](./Scripts)
-  or _external_ - which is a string uniquely identifying the sender. The latter can be used by 
-  a sender-system that doesn't fit into Evennia's normal typeclass-system. 
-  While most systems expect a single sender, it's possible to have any number of them.
-- `receivers` - these are the ones to see the Msg. These are again one of
-  [Account](./Accounts), [Object](./Objects) or [Script](./Scripts). It's in principle possible to have
-  zero receivers but most usages of Msg expects one or more.
-- `header` - this is an optional text field that can contain meta-information about the message. For 
-  an email-like system it would be the subject line. This can be independently searched, making 
+- `senders` - there must always be at least one sender. This is a set of
+- [Account](./Accounts), [Object](./Objects), [Script](./Scripts)
+  or `str` in any combination (but usually a message only targets one type).
+  Using a `str` for a sender indicates it's an 'external' sender and
+  and can be used to point to a sender that is not a typeclassed entity. This is not used by default
+  and what this would be depends on the system (it could be a unique id or a
+  python-path, for example). While most systems expect a single sender, it's
+  possible to have any number of them.
+- `receivers` - these are the ones to see the Msg. These are again any combination of
+  [Account](./Accounts), [Object](./Objects) or [Script](./Scripts) or `str` (an 'external' receiver).
+  It's in principle possible to have zero receivers but most usages of Msg expects one or more.
+- `header` - this is an optional text field that can contain meta-information about the message. For
+  an email-like system it would be the subject line. This can be independently searched, making
   this a powerful place for quickly finding messages.
 - `message` - the actual text being sent.
 - `date_sent` - this is auto-set to the time the Msg was created (and thus presumably sent).
 - `locks` - the Evennia [lock handler](./Locks). Use with `locks.add()` etc and check locks with `msg.access()`
   like for all other lockable entities. This can be used to limit access to the contents
   of the Msg. The default lock-type to check is `'read'`.
-- `hide_from` - this is an optional list of [Accounts](./Accounts) or [Objects](./Objects) that 
+- `hide_from` - this is an optional list of [Accounts](./Accounts) or [Objects](./Objects) that
   will not see this Msg. This relationship is available mainly for optimization
   reasons since it allows quick filtering of messages not intended for a given
   target.